Method and device for encoding a HDR picture

ABSTRACT

A method for coding an HDR picture is disclosed that includes mapping the HDR picture to obtain a SDR picture, determining color correcting parameters, color correcting said SDR picture responsive to said color correcting parameters, and encoding the color corrected SDR picture in a stream. In the method, determining color correcting parameters includes determining an intermediate SDR picture so that at least one color appearance value is preserved between said HDR picture and said intermediate SDR picture, and determining the color correcting parameters from said intermediate SDR picture and from said SDR picture.

1. REFERENCE TO RELATED EUROPEAN APPLICATIONS

This application claims priority from European Application No.15306816.8, entitled “Method and Device for Encoding a HDR Picture,”filed on Nov. 16, 2015, and European Application No. 16305102.2,entitled “Method and Device for Encoding a HDR Picture, filed on Feb. 1,2016, the contents of which are hereby incorporated by reference in itsentirety.

2. TECHNICAL FIELD

In the following, a method and a device for encoding a HDR picture aredisclosed.

3. BACKGROUND ART

For distributing HDR (English acronym of “High Dynamic Range”) videocontent, it is known to first map the HDR video content to obtain avideo content of lower dynamic range, also referred to as a SDR (Englishacronym of “Standard Dynamic Range”) video content. The document ISO/IECJTC1/SC29/WG11 MPEG2015/M37285 entitled Candidate Test Model for HEVCextension for HDR and WCG video coding and published in October 2015discloses a method for encoding HDR and also Wide Gamut video contents.FIG. 1 depicts a simplified flowchart of the method for encoding a HDRpicture as disclosed in the document M37285. The method begins at stepS100. At step S102, a HDR picture is accessed. At step S104, theaccessed HDR picture is mapped to obtain a SDR picture representation ofthe HDR picture, i.e. its dynamic range is reduced. In step S106, theobtained SDR picture is color corrected. The color correction, based ona luma-dependent chroma scaling, aims at controlling the color shift dueto the previous mapping step. It modifies each chroma sample. In stepS108, the color corrected SDR picture is encoded for example using aHEVC encoder.

FIG. 2 depicts a flowchart of a method for mapping the HDR picture toobtain a SDR picture as disclosed in the document M37285. This methodmay be used in the step S104 of the method depicts on FIG. 1.

In step S1042, an inverse EOTF function (EOTF stands for“Electro-Optical Transfer Function”) is applied on the HDR picture toobtain a non-linear HDR signal. In step S1044, the non-linear HDR signalis color transformed, if represented with RGB color values, to obtain aYUV signal (i.e. a signal in a YCbCr color space). In step S1046, theYUV signal is transformed from 444 to 420 format, i.e. the chroma isdownsampled. In step S1048, the 420 YUV values are quantized intointeger values. In step S1050, an ATF function (ATF stands for “AdaptiveTransfer Functionality”) is applied on Y. ATF functionality aims atadjusting dynamic range in the YCbCr domain.

FIG. 3 depicts a flowchart of a method for color correcting the SDRpicture. This method may be used in step S106 of the method depicts onFIG. 1. In a step S1060, two mono-dimensional functions β_(0,U) andβ_(0,V) are obtained or determined. As an example, β_(0,U) and β_(0,V)are represented using Look-Up Tables (LUTs). These functions/parametersmake it possible to control the color saturation of the SDR signal.Possibly, only one function is used for the two chroma components. Inthe document M37285, the functions β_(0,U) and β_(0,V) are referred toas β_(Cb) and β_(Cr) respectively. In step S1062, the chroma samples Uand V of the SDR picture are modified according to the followingequations: U=u/β_(0,U)[Y] and V=v/β_(0,v)[Y]. Color correction divideseach Chroma component Cb and Cr, noted U and V here, respectively by afactor β_(0,U)[Y_(coloc)] and β_(0,V)[Y_(coloc)] which depend on theluma signal. More precisely, the luma signal considered here is the lumacomponent value at a spatial position that is co-located with theconsidered Chroma sample. It is noted Y_(coloc) in the following. In thecase of a 420 signal, the luma component may be averaged on 4 samples tofind the spatial position that is co-located with the considered Chromasample. The functions β_(0,U)( ) and β_(0,V)( ) may be modeled using 2look-up-tables of dimension (2^(BitDepthY)−1) (BitDepthY being thebit-depth of the luma samples). They can be obtained using manualtuning, to enable artistic control of the SDR rendering.

There is thus a need to determine the β_(0,U)( ) and β_(0,V)( ) so thatthe SDR picture is perceptually of good quality.

4. BRIEF SUMMARY

A method for coding an HDR picture is disclosed that comprises:

-   -   mapping the HDR picture to obtain a SDR picture;    -   determining color correcting parameters;    -   color correcting the SDR picture responsive to the color        correcting parameters; and    -   encoding the color corrected SDR picture in a stream.

In a specific embodiment, determining color correcting parameterscomprises:

-   -   determining an intermediate SDR picture so that at least one        color appearance value is preserved between the HDR picture and        the intermediate SDR picture;    -   determining the color correcting parameters from the        intermediate SDR picture and from the SDR picture.

A coding device for coding a HDR picture is disclosed that comprises:

-   -   means for mapping the HDR picture to obtain a SDR picture;    -   means for determining color correcting parameters;    -   means for color correcting the SDR picture responsive to the        color correcting parameters; and    -   means for encoding the color corrected SDR picture in a stream.

In a specific embodiment, the means for determining color correctingparameters comprises:

-   -   means for determining an intermediate SDR picture so that at        least one color appearance value is preserved between the HDR        picture and the intermediate SDR picture; and    -   means for determining the color correcting parameters from the        intermediate SDR picture and from the SDR picture.

A coding device comprising a communication interface to access a HDRpicture is disclosed. The coding device further comprises at least oneprocessor configured to:

-   -   map the accessed HDR picture to obtain a SDR picture;    -   determine color correcting parameters;    -   color correct the SDR picture responsive to the color correcting        parameters; and    -   encode the color corrected SDR picture in a stream.

According to a specific embodiment, to determine color correctingparameters comprises:

-   -   to determine an intermediate SDR picture so that the hue is        preserved between the HDR picture and the intermediate SDR        picture;    -   to determine the color correcting parameters from the        intermediate SDR picture and from the SDR picture.

A non-transitory computer readable medium with instructions storedtherein is disclosed. The instructions, upon execution, instruct atleast one processor to:

-   -   map the accessed HDR picture to obtain a SDR picture;    -   determine color correcting parameters;    -   color correct the SDR picture responsive to the color correcting        parameters; and    -   encode the color corrected SDR picture in a stream.

In a specific embodiment, to determine color correcting parameterscomprises:

-   -   determining an intermediate SDR picture so that the hue is        preserved between the HDR picture and the intermediate SDR        picture;    -   determining the color correcting parameters from the        intermediate SDR picture and from the SDR picture.

The following embodiments and variants applies to all method, device,non-transitory computer readable medium, etc.

According to a specific characteristic, the color appearance value is ahue or a saturation.

In another specific embodiment, determining an intermediate SDR pictureso that a color appearance value is preserved comprises for a currentpixel:

-   -   linearizing a SDR luma value of the current pixel in the        obtained SDR picture;    -   obtaining a corresponding linearized HDR luminance value from        the HDR picture;    -   determining a ratio linking the linearized SDR and HDR luminance        values;    -   determining linear SDR RGB values from the accessed HDR picture        and the ratio; and    -   determining non-linear chroma values from the determined linear        SDR RGB values, the luma value of the intermediate SDR picture        for the current pixel being equal to the luma value of the        obtained SDR picture.

In yet another non-limiting embodiment, determining the color correctingparameters from the intermediate SDR picture and from the SDR picturecomprises minimizing a distance between chroma components of the SDRpicture color corrected and corresponding chroma components of theintermediate SDR picture inside a considered luma sub-range.

In a specific embodiment, color correcting the SDR picture responsive tothe color correcting parameters comprises dividing each color componentof the SDR picture by a color correcting parameter.

5. BRIEF SUMMARY OF THE DRAWINGS

FIG. 1 depicts a simplified flowchart of the method for encoding a HDRpicture according to the prior art;

FIG. 2 depicts a flowchart of a method for mapping the HDR picture toobtain a SDR picture according to the prior art;

FIG. 3 depicts a flowchart of a method for color correcting a SDRpicture according to the prior art;

FIG. 4 represents an exemplary architecture of a transmitter configuredto encode a HDR picture in a stream according to a non-limitingembodiment;

FIG. 5 represents a flowchart of a method for encoding a HDR picture ina stream according to a specific and non-limitative embodiment;

FIG. 6 represents details of FIG. 5;

FIG. 7 represents details of FIG. 5; and

FIG. 8 represents details of FIG. 5.

6. DETAILED DESCRIPTION

It is to be understood that the figures and descriptions have beensimplified to illustrate elements that are relevant for a clearunderstanding of the present principles, while eliminating, for purposesof clarity, many other elements found in typical encoding and/ordecoding devices. It will be understood that, although the terms firstand second may be used herein to describe various color components,these color components should not be limited by these terms. These termsare only used to distinguish one color component from another. Forexample, a first color component could be termed “a component” or “asecond color component”, and, similarly, a second color component couldbe termed “another component” or “a first color component” withoutdeparting from the teachings of the disclosure.

FIG. 4 represents an exemplary architecture of a transmitter 100configured to encode a HDR picture in a stream according to anon-limiting embodiment.

The transmitter 100 comprises one or more processor(s) 1000, which maycomprise, for example, a CPU, a GPU and/or a DSP (English acronym ofDigital Signal Processor), along with internal memory 1030 (e.g. RAM,ROM, and/or EPROM). The transmitter 100 comprises one or morecommunication interface(s) 1010, each adapted to display outputinformation and/or allow a user to enter commands and/or data (e.g. akeyboard, a mouse, a touchpad, a webcam); and a power source 1020 whichmay be external to the transmitter 100. The transmitter 100 may alsocomprise one or more network interface(s) (not shown). Encoder module1040 represents the module that may be included in a device to performthe coding functions. Additionally, encoder module 1040 may beimplemented as a separate element of the transmitter 100 or may beincorporated within processor(s) 1000 as a combination of hardware andsoftware as known to those skilled in the art.

The HDR picture or at least one block of the HDR picture may be obtainedfrom a source. According to different embodiments, the source can be,but is not limited to:

-   -   a local memory, e.g. a video memory, a RAM, a flash memory, a        hard disk;    -   a storage interface, e.g. an interface with a mass storage, a        ROM, an optical disc or a magnetic support;    -   a communication interface, e.g. a wireline interface (for        example a bus interface, a wide area network interface, a local        area network interface) or a wireless interface (such as a IEEE        802.11 interface or a Bluetooth interface); and    -   an image capturing circuit (e.g. a sensor such as, for example,        a CCD (or Charge-Coupled Device) or CMOS (or Complementary        Metal-Oxide-Semiconductor)).

According to different embodiments, the stream may be sent to adestination. As an example, the stream is stored in a remote or in alocal memory, e.g. a video memory or a RAM, a hard disk. In a variant,the stream is sent to a storage interface, e.g. an interface with a massstorage, a ROM, a flash memory, an optical disc or a magnetic supportand/or transmitted over a communication interface, e.g. an interface toa point to point link, a communication bus, a point to multipoint linkor a broadcast network.

According to an exemplary and non-limiting embodiment, the transmitter100 further comprises a computer program stored in the memory 1030. Thecomputer program comprises instructions which, when executed by thetransmitter 100, in particular by the processor 1000, enable thetransmitter 100 to execute the method described with reference to anyFIGS. 5 to 8. According to a variant, the computer program is storedexternally to the transmitter 100 on a non-transitory digital datasupport, e.g. on an external storage medium such as a HDD, CD-ROM, DVD,a read-only and/or DVD drive and/or a DVD Read/Write drive, all known inthe art. The transmitter 100 thus comprises a mechanism to read thecomputer program. Further, the transmitter 100 could access one or moreUniversal Serial Bus (USB)-type storage devices (e.g., “memory sticks.”)through corresponding USB ports (not shown).

According to exemplary and non-limiting embodiments, the transmitter 100can be, but is not limited to:

-   -   a mobile device;    -   a communication device;    -   a game device;    -   a tablet (or tablet computer);    -   a laptop;    -   a still image camera;    -   a video camera;    -   an encoding chip or encoding device;    -   a still image server; and    -   a video server (e.g. a broadcast server, a video-on-demand        server or a web server).

FIG. 5 represents a flowchart of a method for encoding a HDR picture ina stream according to a specific and non-limitative embodiment. On FIG.5, the modules are functional units, which may or not be in relationwith distinguishable physical units. For example, these modules or someof them may be brought together in a unique component or circuit, orcontribute to functionalities of a software. A contrario, some modulesmay potentially be composed of separate physical entities. The apparatuswhich are compatible with the disclosure are implemented using eitherpure hardware, for example using dedicated hardware such ASIC or FPGA orVLSI, respectively “Application Specific Integrated Circuit”,“Field-Programmable Gate Array”, “Very Large Scale Integration”, or fromseveral integrated electronic components embedded in a device or from ablend of hardware and software components.

The method begins at step S200. At step S202, the transmitter 100 accessa HDR picture, e.g. a HDR picture of RGB components. At step S204, thetransmitter maps the accessed HDR picture to obtain a SDR picture, e.g.a YUV or YCbCr SDR picture. As an example, a simple inverse EOTFfunction, e.g. the known PQ EOTF, may be applied followed by a colorspace conversion to generate a YCbCr picture. In yet another variant,the steps S1042 to S1050 disclosed with reference to FIG. 2 may be usedto map the RGB HDR picture to obtain a YUV SDR picture.

It will be appreciated, however, that the present principles are notrestricted to these specific methods for mapping a HDR picture in orderto obtain a SDR picture. Indeed, any method making it possible to reducethe dynamic range of a HDR picture is suitable. In step S207, theobtained SDR picture is color corrected. More precisely, its colorcomponents, e.g. its chroma samples such as UV or CbCr, are modified sothat the perceptual quality of the color corrected SDR picture isincreased. The step S207 comprises a first step S2070 and a second stepS2076.

In the step S2070, the transmitter determines color correctingparameters β_(0,U)( ) and β_(0,V)( ). FIG. 6 details the step S2070. Itcomprises a step S2072 and a step S2074. In the step S2072, thetransmitter determines an intermediate SDR picture with preserved colorappearance, e.g. the hue, with respect to the accessed HDR picture. Tothis aim, it is sufficient to ensure that the proportion between RGBvalues is kept unchanged between the accessed HDR picture and theintermediate SDR picture in the linear RGB color space. In thisintermediate SDR picture the luma is not modified, i.e. the luma of theintermediate SDR picture is set equal to the luma of the SDR pictureobtained at step S204. The step S2072 is detailed on FIG. 7. In a stepS3000, a loop over the pixels of the obtained SDR picture begins. In astep S3002, the SDR luma value Y of a current pixel is linearized. Thelinearized value is denoted LSDR. A luminance value is said to be linearwhen it is proportional to the quantity of light received by a captor. Anon-linear value is usually obtained after application of a correctioncurve, e.g. PQ EOTF, gamma correction, etc. In the literature, the wordsluma and chroma are used in a non-linear domain while the wordsluminance and chrominance are used in the linear domain. For instance,an inverse gamma correction of specification ITU-R BT2020 may be used.An approximation of this inverse gamma correction may be employed asfollows:

$L_{SDR} = \left( \frac{Y}{2^{BitDethY} - 1} \right)^{1/0.45}$

In step S3004, a corresponding HDR linearized luminance value L_(HDR) isobtained. The linearized luminance value L_(HDR) is either obtaineddirectly from a memory or is determined from corresponding RGB valuesaccording to the following equation:

$L_{HDR} = {M_{1X\; 3}\begin{pmatrix}R_{HDR} \\G_{HDR} \\B_{HDR}\end{pmatrix}}$

The matrix M_(1×3) is for example provided by specification ITU-R BT2020for a RGB2020 to YCbCr ad-hoc 3×3 color space conversion.

In step S3006, a ratio w linking the SDR and HDR linearized luminancevalues for the current pixel is determined as follows:w=L_(SDR)/L_(HDR).

In step S3008, the linear SDR RGB values denoted

$\quad\begin{pmatrix}R_{SDR} \\G_{SDR} \\B_{SDR}\end{pmatrix}$for the current pixel are determined from the accessed HDR picture andthe ratio w as follows:

$\begin{pmatrix}R_{SDR} \\G_{SDR} \\B_{SDR}\end{pmatrix} = {w \cdot \begin{pmatrix}R_{HDR} \\G_{HDR} \\B_{HDR}\end{pmatrix}}$where

$\quad\begin{pmatrix}R_{HDR} \\G_{HDR} \\B_{HDR}\end{pmatrix}$are the linear RGB values for the co-located pixel in the accessed HDRpicture. Finally, in step S3010, the non-linear SDR chroma values (UV orCbCr) are determined from the linear SDR RGB values. The operationconsists in SDR gamma correction (approximated by a power of 0.45 here)followed by the RGB to YCbCr color space conversion 3×3 matrix. Since Yis not modified, the first line of the 3×3 matrix is discarded.

$\begin{pmatrix}U_{int} \\V_{int}\end{pmatrix} = {M_{2 \times 3} \cdot \begin{pmatrix}R_{SDR}^{0.45} \\G_{SDR}^{0.45} \\B_{SDR}^{0.45}\end{pmatrix}}$

The current pixel in the intermediate picture has thus the followingvalues: (Y U_(int) V_(int)). The steps S3002 to S3010 are iterated untilall pixels of a picture region, of a whole picture or of a group ofpictures are processed. Indeed, the method may apply on successivepictures of a video, or on only a portion of a picture.

At step S3012, the method ends the loop over the pixels.

In the step S2074, the transmitters determines the color correctingfunctions/parameters from the intermediate picture (Y, U_(int),V_(int))and from the SDR picture (Y,U,V) obtained at step S204, or from regionsof such pictures or from successive intermediate pictures and theircorresponding SDR pictures. FIG. 8 details the step S2074.

To this aim, the luma range is divided in a plurality of subranges. In astep S4000, a loop over the subranges begins. In step S4002, the firstparameter/function β_(0,U)[ ] is determined for the current subrangedenoted currSliceY as follows:

${\beta_{0,U}\lbrack{currSliceY}\rbrack} = {\underset{\beta_{0}}{{Arg}\;\min}\left( {\overset{N}{\sum\limits_{\underset{{Y{\lbrack i\rbrack}}\epsilon\;{currSliceY}}{i = 1}}}\;\left( {{U_{int}\lbrack i\rbrack} - \frac{U\lbrack i\rbrack}{\beta_{0}}} \right)^{2}} \right)}$

In a step S4004, the second parameters/function β_(0,V)[ ] is determinedfor the current subrange denoted currSliceY as follows:

${\beta_{0,V}\lbrack{currSliceY}\rbrack} = {\underset{\beta_{0}}{{Arg}\;\min}\left( {\sum\limits_{\underset{{Y{\lbrack i\rbrack}}\epsilon\;{currSliceY}}{i = 1}}^{N}\;\left( {{V_{int}\lbrack i\rbrack} - \frac{V\lbrack i\rbrack}{\beta_{0}}} \right)^{2}} \right)}$

β_(0,U)[⋅] is for example determined through a least mean squareprocedure, which aims at minimizing the L2 distance between the colorcorrected chroma component U/β_(0,U)[currSlice] and the intermediateU_(int) component values inside the considered Y sub-range. The optimalβ_(0,U)[currSlice] value is given by:

$\begin{matrix}{{\beta_{0,U}\lbrack{currSlice}\rbrack} = \left( \frac{U_{int} \cdot (U)^{t}}{{U}^{2}} \right)^{- 1}} \\{= \left( \frac{\sum\limits_{\underset{{Y{\lbrack i\rbrack}}\epsilon\;{currSliceY}}{i = 1}}^{N}\;\left( {{U_{int}\lbrack i\rbrack} \cdot {U\lbrack i\rbrack}} \right)}{\sum\limits_{\underset{{Y{\lbrack i\rbrack}}\epsilon\;{currSliceY}}{i = 1}}^{N}\;\left( {U\lbrack i\rbrack} \right)^{2}} \right)^{- 1}}\end{matrix}$β_(0,V)[⋅] is computed the same way as β_(0,U)[⋅]·β_(0,V)[⋅] andβ_(0,U)[⋅] may be represented by Look-Up Tables. The steps S4002 andS4004 are iterated until all subranges are processed.

The loop over subranges ends at step S4006.

In a variant, the transmitter determines the color correcting parametersfrom several successive intermediate pictures, for example for a GOP(English acronym of “Group Of Pictures”) or from only picture portions.

Note that with the method of FIG. 8, the β_(0,U)[⋅]· and β_(0,V)[⋅]Look-Up Tables are determined for a limited set of luma values (i.e. theplurality of subranges), corresponding to the Y sub-ranges. A furtherinterpolation step may thus take place after the step S2074. The Look-UpTables β_(0,U)[⋅] and β_(0,V)[⋅] may thus be filled for each value of Yusing for example a simple linear interpolation method.

In the step S2076, the transmitters color corrects the obtained SDRpicture using the determined color correcting parameters. Moreprecisely, the chroma components of the SDR picture obtained at stepS204 are modified as follows:

U=u/β_(0,U)[Y] and V=v/β_(0,v)[Y]. Color correcting thus comprisesdividing each chroma component Cb and Cr, noted U and V here,respectively by a factor β_(0,U)[Y_(coloc)] and β_(0,V)[Y_(coloc)] whichdepend on the luma signal at the same spatial position, whereβ_(0,U)[Y_(coloc)] and β_(0,V)[Y_(coloc)] are the function or LUTsdetermined at step S2074. The encoding method is backward compatible,i.e. the SDR after color correction may be viewed with a good level ofconsistency with the accessed HDR picture. Further the hue is at leastpartially preserved between the accessed HDR picture and the colorcorrected SDR picture.

In a step S208, the color corrected SDR picture is encoded. The colorcorrecting parameters/functions are also encoded for example in SEImessage (SEI stands for “Supplemental Enhancement Information”), in aPicture Parameter Set, or any type of aside metadata. The encoding isfor example compliant with the HEVC video coding standard as disclosedin the document from ITU-T entitled “H.265 series H: Audiovisual andMultimedia systems Infrastructure of audiovisual services—Coding ofmoving video”, MPEG-2, or H.264, etc. It will be appreciated, however,that the present principles are not restricted to these specific methodsfor encoding a picture. Indeed, any method making it possible to encodea picture is suitable. Encoding a picture usually comprises dividing thepicture into picture blocks. The encoding method applies on each block.For a current block, the encoding method usually comprises determining aprediction, determining a residual block by subtracting the predictionblock from the current block. The residual block is then transformedinto frequency coefficients, e.g. by a Discrete Cosine Transform. Thefrequency coefficients are then quantized and the quantized coefficientsare entropy coded. These steps are found in typical encoding and/ordecoding methods.

The method disclosed with the preservation of the hue may be generalizedto the preservation of any appearance value. In another embodiment, thesaturation may be preserved instead of the hue or in addition to thehue.

The implementations described herein may be implemented in, for example,a method or a process, an apparatus, a software program, a data stream,or a signal. Even if only discussed in the context of a single form ofimplementation (for example, discussed only as a method or a device),the implementation of features discussed may also be implemented inother forms (for example a program). An apparatus may be implemented in,for example, appropriate hardware, software, and firmware. The methodsmay be implemented in, for example, an apparatus such as, for example, aprocessor, which refers to processing devices in general, including, forexample, a computer, a microprocessor, an integrated circuit, or aprogrammable logic device. Processors also include communicationdevices, such as, for example, computers, cell phones, portable/personaldigital assistants (“PDAs”), and other devices that facilitatecommunication of information between end-users.

Implementations of the various processes and features described hereinmay be embodied in a variety of different equipment or applications,particularly, for example, equipment or applications. Examples of suchequipment include an encoder, a decoder, a post-processor processingoutput from a decoder, a pre-processor providing input to an encoder, avideo coder, a video decoder, a video codec, a web server, a set-topbox, a laptop, a personal computer, a cell phone, a PDA, and othercommunication devices. As should be clear, the equipment may be mobileand even installed in a mobile vehicle.

Additionally, the methods may be implemented by instructions beingperformed by a processor, and such instructions (and/or data valuesproduced by an implementation) may be stored on a processor-readablemedium such as, for example, an integrated circuit, a software carrieror other storage device such as, for example, a hard disk, a compactdiskette (“CD”), an optical disc (such as, for example, a DVD, oftenreferred to as a digital versatile disc or a digital video disc), arandom access memory (“RAM”), or a read-only memory (“ROM”). Theinstructions may form an application program tangibly embodied on aprocessor-readable medium. Instructions may be, for example, inhardware, firmware, software, or a combination. Instructions may befound in, for example, an operating system, a separate application, or acombination of the two. A processor may be characterized, therefore, as,for example, both a device configured to carry out a process and adevice that includes a processor-readable medium (such as a storagedevice) having instructions for carrying out a process. Further, aprocessor-readable medium may store, in addition to or in lieu ofinstructions, data values produced by an implementation.

As will be evident to one of skill in the art, implementations mayproduce a variety of signals formatted to carry information that may be,for example, stored or transmitted. The information may include, forexample, instructions for performing a method, or data produced by oneof the described implementations. For example, a signal may be formattedto carry as data the rules for writing or reading the syntax of adescribed embodiment, or to carry as data the actual syntax-valueswritten by a described embodiment. Such a signal may be formatted, forexample, as an electromagnetic wave (for example, using a radiofrequency portion of spectrum) or as a baseband signal. The formattingmay include, for example, encoding a data stream and modulating acarrier with the encoded data stream. The information that the signalcarries may be, for example, analog or digital information. The signalmay be transmitted over a variety of different wired or wireless links,as is known. The signal may be stored on a processor-readable medium.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made. For example,elements of different implementations may be combined, supplemented,modified, or removed to produce other implementations. Additionally, oneof ordinary skill will understand that other structures and processesmay be substituted for those disclosed and the resulting implementationswill perform at least substantially the same function(s), in at leastsubstantially the same way(s), to achieve at least substantially thesame result(s) as the implementations disclosed. Accordingly, these andother implementations are contemplated by this application.

What is claimed is:
 1. A method for coding a HDR picture comprising:mapping the HDR picture to obtain a SDR picture; determining colorcorrecting parameters; color correcting said SDR picture responsive tosaid color correcting parameters; encoding the color corrected SDRpicture and the color correcting parameters in a backward compatiblestream, so that a corresponding HDR picture can be generated from theSDR picture and the color correcting parameters; wherein the methodoperates in a YCbCr color space; and wherein determining colorcorrecting parameters comprises: determining color correcting parameterscomprises: determining an intermediate reference SDR picture so thatluminance of the intermediate picture is equal to luminance values ofthe obtained SDR picture and hue is preserved between said HDR pictureand said intermediate reference SDR picture; determining the colorcorrecting parameters for Cb and Cr components from said intermediatereference SDR picture and from said obtained SDR picture.
 2. The methodof claim 1, wherein determining an intermediate SDR picture so that thehue is preserved comprises for a current pixel: linearizing a SDRluminance value of said current pixel in the obtained SDR picture;obtaining a corresponding linearized HDR luminance value from said HDRpicture; determining a ratio linking the linearized SDR and HDRluminance values; determining linear SDR RGB values from the accessedHDR picture and the ratio; and determining non-linear chroma values fromthe determined linear SDR RGB values, the luminance value of theintermediate SDR picture for the current pixel being set equal to theluminance value of the obtained SDR picture.
 3. The method according toclaim 1, wherein determining the color correcting parameters from saidintermediate reference SDR picture and from said SDR picture comprises,inside a considered luma sub-range, determining a lookup table for Cbchroma components by minimizing a L2 distance between Cb chromacomponents of said SDR picture color corrected and corresponding Cbchroma components of said intermediate reference SDR picture anddetermining a lookup table for Cr chroma components by minimizing a L2distance between Cr chroma components of said SDR picture colorcorrected and corresponding Cr chroma components of said intermediatereference SDR picture.
 4. The method according claim 1, wherein colorcorrecting said SDR picture responsive to said color correctingparameters comprises dividing each color component of said SDR pictureby a color correcting parameter.
 5. A coding device comprising acommunication interface to access a HDR picture and at least oneprocessor configured to: map the accessed HDR picture to obtain a SDRpicture; determine color correcting parameters; color correct said SDRpicture responsive to said color correcting parameters; encode the colorcorrected SDR picture and the color correcting parameters in a backwardcompatible stream, so that a corresponding HDR picture can be generatedfrom the SDR picture and the color correcting parameters; wherein theoperations are performed in a YCbCr color space and wherein to determinecolor correcting parameters comprises: to determine an intermediatereference SDR picture so that luminance of the intermediate picture isequal to luminance values of the obtained SDR picture and hue ispreserved between said HDR picture and said intermediate reference SDRpicture; to determine the color correcting parameters for Cb and Crcomponents from said intermediate reference SDR picture and from saidobtained SDR picture.
 6. The coding device of claim 5, wherein todetermine an intermediate SDR picture so that the color appearance valueis preserved comprises for a current pixel: linearizing a SDR luminancevalue of said current pixel in the obtained SDR picture; obtaining acorresponding linearized HDR luminance value from said HDR picture;determining a ratio linking the linearized SDR and HDR luminance values;determining linear SDR RGB values from the accessed HDR picture and theratio; and determining non-linear chroma values from the determinedlinear SDR RGB values, the luminance value of the intermediate SDRpicture for the current pixel being set equal to the luminance value ofthe obtained SDR picture.
 7. The device according to claim 5, wherein todetermine the color correcting parameters from said intermediatereference SDR picture and from said SDR picture comprises, inside aconsidered luma sub-range, to determine a lookup table for Cb chromacomponents by minimizing a L2 distance between Cb chroma components ofsaid SDR picture color corrected and corresponding Cb chroma componentsof said intermediate reference SDR picture and to determine a lookuptable for Cr chroma components by minimizing a L2 distance between Crchroma components of said SDR picture color corrected and correspondingCr chroma components of said intermediate reference SDR picture.
 8. Thedevice according to claim 5, wherein color correcting said SDR pictureresponsive to said color correcting parameters comprises dividing eachcolor component of said SDR picture by a color correcting parameter. 9.A non-transitory computer readable medium with instructions storedtherein which, upon execution, instruct at least one processor to: mapthe accessed HDR picture to obtain an SDR picture; determine colorcorrecting parameters; color correct said SDR picture responsive to saidcolor correcting parameters; encode the color corrected SDR picture andthe color correcting parameters in a backward compatible stream, so thata corresponding HDR picture can be generated from the SDR picture andthe color correcting parameters; wherein the operations are performed ina YCbCr color space and wherein to determine color correcting parameterscomprises: to determine an intermediate reference SDR picture so thatluminance of the intermediate picture is equal to luminance values ofthe obtained SDR picture and hue is preserved between said HDR pictureand said intermediate reference SDR picture; to determine the colorcorrecting parameters for Cb and Cr components from said intermediatereference SDR picture and from said obtained SDR picture.